diff --git a/cloudinit/net/network_state.py b/cloudinit/net/network_state.py
index d0d86ce..8f5e9b2 100644
--- a/cloudinit/net/network_state.py
+++ b/cloudinit/net/network_state.py
@@ -689,43 +689,45 @@ class NetworkStateInterpreter(object):
 
 
 def _normalize_subnet(subnet):
-    subnet = copy.deepcopy(subnet)
-    for k in ('address', 'gateway', 'netmask'):
-        if k in subnet and not subnet[k]:
-            del subnet[k]
+    # Prune all keys with None values.
+    normalized_subnet = dict((k, v) for k, v in subnet.iteritems() if v)
 
-    if 'address' in subnet:
-        addr = subnet['address']
+    addr = subnet.get('address')
+    prefix = subnet.get('prefix')
+    netmask = subnet.get('netmask')
+    if addr:
         if "/" in addr:
-            toks = subnet['address'].split("/", 2)
+            toks = addr.split('/', 2)
             subnet['address'] = toks[0]
-            subnet['prefix'] = int(toks[1])
-        elif subnet.get('prefix'):
-            subnet['prefix'] = int(subnet['prefix'])
-        elif subnet.get('netmask'):
-            subnet['prefix'] = mask_to_net_prefix(subnet.get('netmask'))
+            # XXX If prefix is defined by the user but address is also CIDR, we
+            # silently overwrite the user's original prefix here. We should
+            # log a warning about ignoring user-defined prefix and netmask if
+            # present.
+            subnet['prefix'] = toks[1]
+        elif netmask:
+            subnet['prefix'] = mask_to_net_prefix(netmask)
         else:
-            if is_ipv6_addr(subnet['address']):
+            if is_ipv6_addr(addr):
                 subnet['prefix'] = 64
             else:
                 subnet['prefix'] = 24
+    # XXX this 'try' here so we don't have to handle non-ints in 2 places
+    # above int(toks[1]) and int(subnet['prefix'])
+    if prefix:
+        try:
+            prefix = int(prefix)
+        except ValueError:
+            raise TypeError('prefix {} is not an integer'.format(prefix))
+               
 
     #if is_ipv6_addr(addr):
     #    subnet['netmask'] = net_prefix_to_ipv6_mask(subnet['prefix'])
     #else:
     #    subnet['netmask'] = net_prefix_to_ipv4_mask(subnet['prefix'])
-    if 'netmask' in subnet:
-        del subnet['netmask']
-
-    if 'addr' in subnet and not subnet['addr']:
-        del subnet['addr']
-
-    routes = subnet.get('routes')
-    if not routes:
-        subnet['routes'] = []
-    else:
-        subnet['routes'] = [_normalize_route(r) for r in subnet['routes']]
+    # Remove netmask and 
+    subnet.pop('netmask', None)
 
+    subnet['routes'] = [_normalize_route(r) for r in subnet.get('routes', [])]
     return subnet